1. Field of the Invention
The present invention relates to an image forming apparatus of the tandem type for sequentially transferring a plurality of images to the same transfer medium, and specifically relates to an improvement of the art for controlling transfer output dependent on transfer timing.
2. Description of the Related Art
In recent years various types of color image forming apparatuses have been developed in response to demand for color copiers and color printers; an example of such an image forming apparatus is shown in FIG. 14.
FIG. 14 briefly shows the general construction of a tandem type image forming apparatus.
As shown in the drawing, the tandem type image forming apparatus provides at predetermined intervals photosensitive drums 401C-401K image processing units 400C-400K used for color image formation of cyan (C), magenta (M), yellow(Y), and black (K) arrayed from the upstream side in the sheet transport direction (hereinafter referred to simply as "upstream side") to the downstream side in the transport direction (hereinafter referred to simply as "downstream side") of transport belt 201 circumscribing a drive roller 202 and a driven roller 203 (hereinafter the reproduced colors of cyan (C), magenta (M), yellow (Y), and black (K) are represented simply by C, M, Y, and K, and the reference numbers of components related to said reproduced colors have the symbol C, M, Y, or K appended thereto).
Before being optically exposed by a light beam emitted from an optical scanning unit not shown in the drawing, the surfaces of photosensitive drums 401C-401K are uniformly charged by chargers 404C-404K after residual toner is removed from said surfaces by cleaners 402C-402K and residual charge is removed therefrom by exposure from eraser lamps 403C-403K, and when the uniformly charged surfaces are exposed by the aforesaid light beam, electrostatic latent images are formed on the respective surfaces of photosensitive drum 401C-401K. These electrostatic latent images are developed by toners of colors C-K supplied from developing devices 405C-405K, thereby forming C-K toner images on the surfaces of photosensitive drums 401C-401K, respectively.
On the other hand, before recording member S such as a paper sheet or OHP sheet is placed on transport belt 201, the residual charge remaining on transport belt 201 is discharged by dischargers 801 and 802, residual adhered toner is removed from the surface of said belt by belt cleaner 900 so as to maintain the surface in a clean state, then transfer sheet S is fed from sheet guide 901 to transport belt 201, and said transfer sheet S is adhered to transport belt 201 by a charge applied between driven roller 203 and opposing roller 501 at the adhere position of recording sheet adhere unit 500, and said transfer sheet S is then fed to the transfer positions of image processing units 400C-400K synchronously with the image forming operation of the aforesaid photosensitive drums 401C-401K. When the charge is imparted between photosensitive drums 401C-401K and the transfer rollers 406C-406K at the transfer position with the transfer sheet S in the aforesaid adhered state, the toner images C-K formed on photosensitive drums 401C-401K are sequentially transferred onto transfer sheet S so as to be superimposed one upon another thereon without transfer sheet S wrapping around said photosensitive drums 401C-401K. The transfer sheet S carrying the toner images is separated from the transport belt 201 via discharge from separation charger 701, and the toner particles on the surface of the transfer member are fused to the surface of the sheet via fixing device 902.